This invention relates to an electric fence energiser.
The term energiser in connection with electric fencing is commonly used to indicate a generator of a high voltage output which can be connected to an electric fence to electrify the fence. Other terms used to identify this piece of electric fencing equipment include fence controller, electrifier, charger, pulse generator and the like.
Electric fencing is widely used for the control of livestock animals, game, perimeter security installations and possible other uses. In such installations, the energiser performs the function of supplying a high voltage to one or more conductors of the electric fence with the aim of providing an electric shock to an animal or a person touching one or more of the conductors. The high voltage on the fence conductors may be present in the form of intermittent pulses of short duration, or as a continuous AC or DC voltage.
A significant proportion of energisers provide intermittent high voltage pulses of short duration to the electric fence. Other energisers provide continuous high voltage DC or AC. The reasons for the popularity of the pulsed type of energiser are many. One of the reasons is that for most practical electric fence installations the pulsed type of energiser is capable of providing a more powerful electric shock than other types of energisers, and thus provides a better deterrent for animals or persons attempting to cross the barrier formed by the electric fence.
Currently there are three main types of electric fence energisers used in the field of electric fencing. One type, which accounts for the majority of energisers, is a capacitor discharge model with a step-up transformer. This type of energiser operates by discharging one or more energy storage capacitors through a primary winding on the transformer. The secondary winding, which typically has a greater number of turns than the primary winding, thereby transforms the voltage that is imposed across the primary winding to a higher voltage. The secondary winding is usually directly electrically connected to the electric fence.
A semiconductor switching device is held in the off (blocking) state to allow charging of the energy storage capacitor(s). When charging is complete the switching device is placed in the on (conducting) state to rapidly connect the capacitor to the primary winding of the transformer thereby allowing a rapid discharge and production of a high voltage pulse.
There are a number of reasons why this type of energiser has become the most popular. These include:—                Energy storage capacitors with rated voltages between 250V DC and 1200V DC are mass-produced, are low cost and readily available.        Thyristor switching devices with rated voltages between 400V DC and 1200V DC are also mass-produced, low cost and readily available. In addition, in a high-energy energiser a large current flows through the switching device, the capacitor and the transformer primary winding during the pulse (in the order of several hundreds of amperes to over a thousand), which can be tolerated by the same low cost thyristors.        A peak pulse voltage of 1200V is generally considered insufficient (too low) to effectively deter animals (livestock) from crossing a barrier formed by an electric fence. As a general rule, pulses with a peak voltage of 3000V and higher are considered adequate.        Electric fence energisers powered from the AC mains and some models powered by battery generally have an electrical safety isolating barrier between the AC mains terminals and the fence terminals. This barrier is mandatory in the interest of safety. The step-up transformer is constructed to perform a dual function:                    (1) To increase the voltage from between 400 . . . 1200V to more than 3000V as required for effectiveness of the electric fence barrier.            (2) The mandatory electrical isolating barrier is most easily constructed between the primary and the secondary winding of the step-up transformer.                        
A second type of electric fence energiser is the inductive discharge model in which the step-up transformer functions as the energy storage device as well as the means of increasing the pulse output voltage to the desired level. Thus in this type of energiser energy is stored in the magnetic field (iron core) of the step-up transformer by allowing a current to build up in the primary winding of the transformer. When this current is abruptly interrupted (typically by using a semiconductor switching device such as a power MOSFET or a BJT), a high voltage pulse is developed across the windings of the transformer.
This type of energiser is not very popular mostly because it is limited to maximum power levels that are considerably lower than what can be achieved using the capacitor discharge topology. In addition, controlling the maximum pulse voltage may require additional components such as high voltage varistors (MOVs).
The third type of energiser is a DC fence charger, typically formed by placing a constant high DC voltage on the fence conductors by means of a low current (high impedance) voltage multiplier circuit. These types of energisers are predominantly used in North America. The charger typically is constructed using a “capacitor and diode” voltage multiplier chain, rectifying and multiplying mains input voltage up to the desired output voltage. Either the output of the chain is connected to the fence conductors via relatively high impedance, or the mains input is connected to the chain via similarly high impedance. The high impedance is mandatory for this type of energiser to ensure safety.
This type of energiser is limited to very low power levels due to the required high impedance.
Both capacitor discharge type and inductive discharge type energisers tend to be wasteful of energy for many load conditions. In an energiser that may be considered state-of-the-art about 20% or more of the stored energy is lost in the electronic components forming the pulse generating circuit, even under the most favourable load conditions. For loads other than the most favourable value the loss increases and reaches 100% for many designs under open-circuit load conditions. Whilst such energy loss is often not of concern for low- and medium-energy energisers, energy loss in internal circuits can become a problem for high-energy energisers. In addition, if an energiser is supplied by an energy-limited supply such as a battery or a solar panel it is desirable to minimise energy losses to maximise battery life and/or to minimise size and cost of the battery and/or solar panel.
One of the components responsible for a significant amount of energy loss is the step-up transformer employed in both the capacitor discharge- and the inductive discharge-type of energiser. Especially in high-energy energisers the step-up transformer is a major source of energy loss due to resistive losses in the copper windings, hysteretic and eddy current losses in the magnetic core and poor inductive coupling of the windings due to saturation of the magnetic core material.
It is possible to improve the efficiency of the step-up transformer by configuring the transformer as what is commonly known as an auto-transformer, wherein the primary and secondary windings are not electrically isolated. However, whilst the auto-transformer mostly offers an improvement by way of better coupling between the windings, the other losses associated with step-up transformers remain largely or entirely the same and the advantage of improved coupling may be partially lost if the magnetic material of the core becomes saturated during the pulse.
Energisers generally comply with international and national safety standards. In particular, limits are applied to the minimum pulse interval duration, the maximum amount of energy and/or the maximum magnitude and duration of the electric current per pulse that an energiser is allowed to supply to certain standard load impedances connected to the energiser. Although capacitor discharge type energisers can be easily made to comply with such safety standards, such energisers still have a limited amount of control over the three pulse parameters energy, current and duration. Many designs overcome some of the limitations by regulating the voltage to which the energy storage capacitor is charged, and some designs also provide the ability to discharge more than one capacitor or bank of capacitors, thereby attempting to maximise pulse voltage for a wider range of load impedances than is possible with just a single energy storage capacitor or bank of capacitors.
The step-up transformer used in conventional energiser designs places a severe restriction on the maximum pulse width that can be achieved, because the magnetic core material tends to become saturated for longer pulse durations.
A capacitor discharge energiser with a step-up transformer generates a current in the primary winding that can reach hundreds of amperes for low- to medium-energy designs and may reach thousands of amperes for high-energy designs. To be able to control and switch currents of this magnitude the preferred device is a thyristor, also commonly known as a silicon controlled rectifier SCR. Sometimes a triac is used.
A limitation of a thyristor and triac is that it is difficult to turn the device off (i.e. revert the device to the non-conducting state) once it is placed in the conducting state. In a practical situation this means that most or all of the energy stored in the energy storage device is transferred and/or dissipated before the thyristor or triac returns to the non-conducting state. The difficulty in turning off the switching device therefore is the reason, in most current energiser designs, for a limitation on the minimum pulse duration that the energiser can produce.
Many attempts have been made to overcome problems inherent with electric fence energisers of the aforementioned type. Many of these attempts have focused on an energiser which can be controlled so as to vary the output essentially in response to load on the electric fence. One approach has been to incorporate multiple energy storage capacitors and then use a control circuit to allow one or more of the capacitors to discharge, the number of capacitors being discharged being in response to a sensed load on the fence.
Another approach has been to charge the storage capacitor to a level commensurate with a load sensed on the electric fence so that upon discharge the required energy level is transferred to the fence.
Yet another approach proposes circuitry in which there are a number of storage capacitor/step-up up transformer combinations and control means to trigger one or more of the combinations dependent on the sensed load on the fence line.
All of these approaches have been intended to deal with problems inherent with known constructions of energiser though, more particularly, with capacitor discharge type energisers. By controlling the energy stored in or discharged from the energy storage device(s) using one of the abovementioned methods, the amount of energy output at each discharge can be controlled for either energy conservation or safety purposes. Also, factors such as heat build up in the energiser can be improved.
In all of these approaches the output is controlled by the amount to which the energy storage capacitor is charged or the number of energy storage capacitors which are discharged to create the output pulse.